The present invention relates generally to a torque converter with a friction clutch applicable to automotive automatic power transmissions. More particularly, the invention relates to a slip control for the friction clutch which provides better transition characteristics between hydrodynamic drive and mechanical drive.
U.S. Pat. No. 3,933,031 to Peterson et al and U.S. Pat. No. 4,002,228 to Borman disclose torque converters of automatic power transmissions including friction clutch slip control mechanisms. Both of these slip control mechanisms control friction clutch slippage by controlling working fluid pressure in a disengagement chamber relative to that in an engagement chamber. Pressure control is performed by adjusting the cross-sectional area for introducing pressurized working fluid into the disengagement chamber. In order to adjust the cross-section, a throttling fluid path is defined in communication with the disengagement chamber. The cross-section of the throttling path varies according to the relative movement of a turbine runner serving as an output element of a torque converter and a output shaft.
In these conventional slip controls techniques, the throttling rate changes relatively slowly near the minimum and maximum cross-sections. On the other hand, the rate of change of the throttling rate in the intermediate range is relatively high, where most slip control is practically carried out. As a result, fluid pressure in the disengagement chamber varies at a relatively high rate in the intermediate throttling range. This unavoidably causes hunting of the fluid pressure balance between the disengagement chamber and engagement chamber, and thus between the hydrodynamic drive state and the mechanical drive state. Abrupt clutch release and engagement due to significant changes in the pressure balance between the disengagement chamber and engagement chamber may cause fluctuations in speed and output of the engine serving as the prime mover. This degrades the drivability of the vehicle.